Hafid Ibn Hajar Asqalani (may Allah Almighty have mercy upon him) writes in Nukhbat al-Fikr:

So I say: a khabr either it will have an (1) unlimited number of sources or (2) a limited which is (a) more than two, (b) two or (c) one. The first is mutawatir with its criteria1 which results in necessary knowledge. The second is Mashhur2 and it is also known as mustafidh according to an opinion. The third is Aziz and this is not a requisite for sahih as opposed what some have assumed. The fourth is gharib. All of them other than the first3 is ahad. [Ahad] may be maqbul or mardud. [This is] due to [the ahad being] evidence which is dependent upon the analyses of the state of the narrators as opposed to the first. Thereafter, the gharabat will either be in the origin of the sanad or not; the first is fard mutlaq and the second is fard nisabi which is rarely referred to simply as fard.

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Notes:

• Necessary knowledge (ilm daruri or ilm yaqini) is knowledge which is self-evident (requires no evidence) and understandable to any average person who hears. This is in contrast to evidentiary or convincing knowledge (ilm iktisabi or ilm nazari) which requires evidence to know and a level of expertise to understand (cf Nuzhat al-Nazar lil Asqalani).

• 1See 'The introduction to Mutawatir' for the criteria. Also, read The types of mutawatir and Where does tawatur start? for more details.
• 2This is the definition according to the Shawafi. The Ahnaf consider Mashhur to be sanad which starts of ahad but reaches the level of tawatur by no later than the third generation.
• 3Mutawatir